It is the new Safari updated with 10.13.x-10.13.4, everything is working as it should. I do not notice any slow down or disruption to Safari in all honesty, besides the pop-up banner. This may be a clue that Safari is working as efficiently on your behalf to keep things running smooth behind the scenes.
Ignore the warning if you wish, or excuse it by the x. There is absolutely no user harm either way. I suspect if (like before the new Safari) you did not get the warning, you would be non-the-wiser.
Just because Safari is "made for Mac," it has no control over half baked code a website is running. (Maybe that would be the better warning message.) A resource intense interface. As you can imagine CNN, Facebook run some realtime API's that not all rendering engines interpret in same way. This is by design or there would be no difference or need for different browsers. Think of it this way that warning message keeps people employed. 😉
If this troublesome warning message popped up on every other website—then yes Safari has a problem. It simply is not case, from your post, other post on ASC, or my own user experience.
By all means send Apple Feedback http://www.apple.com/feedback/
Please note I am not an apologist for the Safari.app I just see it as a non-issue. Surely there are bigger fish to fry.